1. Field of the Invention
The present invention relates to a circuit for correction of white pixel defects and an image sensor using the circuit for correction of white pixel defects, and more particularly, to a circuit for correction of white pixel defects for complementing white pixel faults occurring in a pixel section constituted by solid-state imaging elements and to an image sensor using such a circuit for correction of white pixel defects.
2. Description of the Related Art
Currently, CCD (Charge Coupled Device) and CMOS (Complementary Metal Oxide Semiconductor) image sensor are used as solid-state imaging devices for digital cameras, digital video cameras, etc.
CCDs and CMOS image sensors sometimes include a pixel at a particular position on the imaging area that outputs a level higher than a fixed level and always shows a fixed color, because of characteristics of photodiodes, variations of sensors, etc. This phenomenon is called white pixel fault or white pixel stain and is difficult to perfectly eliminate in currently available manufacturing process. Where a dark image is acquired by an image sensor having a white pixel fault, a spot associated with the white pixel fault outputs bright-color data, making the spot extremely noticeable. Conventionally, therefore, a complementation process is performed using data of pixels near the fault spot.
A conventional circuit for correction of white pixel defects for complementing white pixel faults will be explained. FIG. 19 illustrates the configuration of a conventional circuit for correction of white pixel defects and its peripheral circuitry.
Spots where a white pixel fault occurs are peculiar to individual image sensors, and therefore, such spots are previously stored in a storage device 910 for storing white pixel fault spots. A coincidence comparator 920 determines whether or not position information (POS) identifying the position of a pixel coincides with the position of a white pixel fault stored in the white pixel fault spot storage device 910, and supplies the result to a circuit for correction of white pixel defects 930. When notified that a pixel of which the pixel data (DATA) has been input coincides with a pixel associated with the white pixel fault, the circuit for correction of white pixel defects 930 performs a white pixel fault complementation process by using pixel data of nearby pixels. Data which has been subjected to the white pixel fault complementation process is converted to RGB data in an RGB conversion circuit 940 and then output.
Thus, the conventional circuit for correction of white pixel defects requires a storage device for storing white pixel fault spots, giving rise to a problem that the circuitry becomes redundant. A problem also arises in that inspection is needed for the detection of white pixel fault spots.
As seen from the above explanation, the conventional circuit for correction of white pixel defects must be provided with a storage device, such as a register or ROM, for storing previously detected white pixel fault spots. The storage device must have a storage capacity corresponding to an allowable number of white pixel faults to be corrected. Accordingly, increase in the total number of pixels of the image sensor leads to increased capacity and redundancy of the storage device, as well as to enlargement in scale of the circuitry. On the other hand, image sensors having white pixel faults exceeding the allowable number for correction are rejected as defective sensors, and thus if the allowable number for correction is set small to keep the required storage capacity small, a problem arises in that the number of defective products increases. In recent years, products tend to have an increased total number of pixels, and the method using a storage device to complement white pixel faults is reaching its limits of practicality.
In conventional white pixel fault complementation methods, each of previously detected pixels associated with a white pixel fault is complemented using a predetermined coefficient etc. Thus, since the complementation process is carried out for the individual white pixel fault spots independently of one another, the resulting image sometimes looks unnatural.
As a method for removing white pixel faults, median filtering using no redundant storage device is also known, but this method is disadvantageous in that the image resolution lowers.
Further, there is a problem that in order to store in advance white pixel fault spots, inspection needs to be performed for each of image sensor chips to detect white pixel fault spots. Consequently, the inspection step for detecting white pixel fault spots and the step for registering the detected spots must be included in the manufacturing process, making also the manufacturing process redundant.
Thus, the conventional redundant circuit for correction of white pixel defects using a storage device poses a problem in cases where a system using an image sensor or an image sensor itself is miniaturized and integrated as an LSI. Also, where an image sensor is used in portable telephones etc. whose total number of pixels is small, reduction in the resolution of the image sensor gives rise to a problem that the image becomes blurred.